1. Field of the Invention
This invention relates to apparatus and methods for determining the spatial position of an edge of a workpiece, and more particularly to apparatus and methods for determining the spatial relationship between leads extending from an integrated circuit body to others of such leads and to the body, and methods and apparatus for using such position measurements to aid in quality control. In another aspect, this invention relates to devices for different sizes of workpieces to be rotated by a given angle without reconfiguration or adjustment.
2. Description of Related Art
For many applications it is desirable to mechanically determine the position of an edge on a workpiece with repect to a reference position. One such application is quality control, in which the position of an edge of a manufactured product is compared to the position of other edges of the product in order to ensure that the product is within certain specifications. Another application is in process control, in which the same function is performed at an earlier stage in the manufacturing process or at the end of the manufacturing process but in much finer detail. This permits adjustment of manufacturing parameters before the resulting component begins to exceed tolerance, or to warn of failing manufacturing apparatus before it actually fails.
An important application for the present invention is in the field of integrated circuit (IC) manufacture, where it is desirable to ensure the accuracy of the shape and position of the leads on the IC package. In this field, industry standards organizations, such as JEDEC, have promulgated specifications concerning certain types of packages, such as J-lead PLCC and small outline (SOIC) packages. These specifications are relied on by many different segments of the industry, including manufacturers of the printed circuit boards to which the IC's will be affixed, and manufacturers of pick-and-place machines for automatically placing IC's in their proper positions on a printed circuit board at high speeds.
An integrated circuit manufacturer usually has the task of keeping its output within the tolerance specified in the standards. These tolerances are often very tight as written, and in many cases the end users of the integrated circuits are beginning to require them to be even tighter. In addition, it is advantageous to the manufacturer if he can detect yet smaller deviations from the specified nominal positions in order to correct a gradually worsening problem in the manufacturing process before it ever reaches a severity necessary to cause rejection of any manufactured IC's for failure to meet the specified tolerances.
It is important to an IC manufacturer that the system which it uses to inspect IC leads have an accuracy which is as high as possible. The tolerance with which inspection measurements are made determine a region of uncertainty within which it is uncertain whether or not the product meets the tolerances specified in the industry standard. A cautious manufacturer should fail any product in the grey area. The higher the accuracy of the inspection system, therefore, the fewer good products will be rejected due to uncertainty. The quantity of these unnecessarily rejected products is a critical factor in the economic efficiency of such a machine.
The following definitions of terms will help the reader understand the invention and the description of the embodiments which appears below. Some of these terms relate mainly to J-lead IC packages, but it will be understood that the invention is not limited in application to such packages. It can be used not only with other types of electronic component packages, but also with workpieces completely outside the field of integrated circuits or electronics. The dimensions listed below are those that apply for a PLCC package with leads on 0.050" centers on all four sides.
Coplanarity
The distance between the seating plane of a multileaded component and a lead tip which does not contact the seating plane is the coplanarity distance of that lead tip. The seating plane is the plane on which the package will rest, leads directed downward, under the influence of gravity. Coplanarity is specified separately for each lead. The current maximum coplanarity tolerance is -0.004". All coplanarity distances are defined as negative. For a J-lead, which has a rounded tip, the lead tip is defined as the point on the lead at which a tangent to the lead is parallel to the seating plane.
Sweep
The distance between the tips of adjacent leads on one side of a package. The tolerance on sweep is .+-.0.004" with respect to an ideally precise array of pins.
Spread
The distance, in a direction toward or away from the package body, by which the center of a lead tip deviates from a line parallel to the body edge. The tolerance on spread is .+-.0.008" with respect to an ideally precise array of pins.
Tweezing coplanarity
A measure of the coplanarity of the lead faces on a side of the package. A line drawn through the shoulders of the two leads on a subject side of the package which are the farthest from a datum line must not deviate from a line parallel to the datum line by more than .sup..+-. 6 mils. The datum line is defined as the line drawn through the center leads on the two sides of the package which are adjacent to the subject side.
A variety of different devices and methods have been used in the past to make these measurements. One technique is by human observation, a technique which is still in current use. For example, in order to measure coplanarity, a package is placed leads-down on a mirrored surface. The distance between each lead tip and its reflection is then visually inspected with an optical comparator to determine whether all the leads on a given side, and their reflections, are between a pair of reference lines. This technique has many obvious disadvantages including low throughput, low accuracy and high cost. Moreover, many of the specified parameters simply cannot be measured by human observation without detailed analysis requiring an inordinate amount of time to inspect each package.
Another technique often used to inspect the leads on IC packages involves air pressure measurement. An IC package is pressed into contact with the surface of a plate, and the spaces between leads are blocked as well as possible by mechanical means. Air is forced through a hole in the plate into the space between the package body and the plate, and the air flow between the plate and each of the lead tips is measured. This technique has a throughput comparable to that of the human means described above, since operator throughput is still a limitation. Similarly, accuracy and versatility are still a problem. With respect to versatility, it can be seen that an air pressure measurement technique is not readily adaptable for measuring other parameters which are specified in the standards. The technique as described does in a way provide a measurement of sweep and spread, but only to the extent that the sweep or spread affects the coplanarity of the lead tips. It does not provide these measurements independently of coplanarity.
A third technique for inspecting IC package leads is an imaging technique in which one or more video cameras are used to make video images of the leads. An anamorphic lens may be used to compress the image in the sweep direction and expand it in the Coplanarity direction to improve resolution for coplanarity measurements. Coplanarity and sweep measurements are made using four cameras, one directed toward each side of an IC package which rests on a horizontal plane with its leads facing upward. Spread is measured with a fifth camera positioned above the lead tips and directed toward the undersurface (base plane) of the package. It is believed that the measurements made for the four sides individually are interrelated (as they must be in order to determine certain of the parameters, such as coplanarity) through the use of a mechanical alignment procedure in which a calibration component is placed in the machine and the cameras or lens systems physically moved until alignment is achieved.
The imaging technique is a sophisticated technique with high flexibility in its application and relatively high accuracy. The sophisticated image processing algorithms required in this technique, however, render it extremely computationally intensive. Its throughput is limited by the processing power of the computer used to do the calculations. Additionally, an imaging system is not readily reconfigurable to handle different types or sizes of IC packages because any such change would likely require a change to different optic heads adapted for the new package. Imaging systems also suffer from depth-of-field problems and degradation of accuracy due to changes in surface finish or orientation.
A fourth technique is illustrated in FIG. 1. It involves a seating surface 10 having a hole 12 smaller than the size of the part 14 to be tested. A pedestal 16 depends through the hole, which pedestal has a mirror 18 on each of its four sides. The mirrors 18 are angled such that light directed onto them from above the seating surface and passing through the hole 12 will be reflected outward, away from the pedestal, in a known direction. The part 14 under test is inverted and raised until it rests in contact with the seating surface 10, its leads surrounding the hole. The light which is directed onto the mirrors 18 is divided into separate beams, one for each lead, the width of each beam being slightly narrower when it reaches the lead than the lead width, and directed so as to be completely blocked if the lead is positioned properly. If a given lead is not on the seating surface 10, the amount of light which gets past the lead depends on the distance between the surface 10 and the lead tip. The leads are themselves surrounded on all four sides by detectors 20, one for each lead, in such a position that any light which gets past the lead tip will be detected by the corresponding detector 20. Each detector 20 is located behind a light trap 22 consisting of a small aperture 24 and a darkened tunnel 26, to prevent off-axis light from reaching the detector.
The deliverable resolution of inspection devices which use the technique is very good. Since it measures coplanarity directly, however, it lacks the versatility to effectively measure other parameters. The device will fail a part for excess sweep, since anything more than slight misalignment between the lead and the beam will permit light to reach the detector from around the side of the lead. This is not very helpful if the manufacturer desires to quickly correct the problem in the manufacturing process which is causing the defect, however, because the inspection device cannot determine whether the defect is in coplanarity or sweep. The inclusion of this sweep detection capability also places a premium on the accurate placement of the part under test. If it is placed only slightly out of position, all the leads will be seen to have excess sweep and the part will fail. The need for accurate placement of parts under test limits the throughput which may be achieved.
In U.S. Pat. No. 4,553,843 to Langley and Brekka, there is disclosed a light blocking technique which may be used to inspect the leads on a body for conformance with a specified standard. The technique involves placing the IC package in a track or on a conveyor belt and moving it through a test area with a known velocity or acceleration. One or more light source-detector pairs are located in the test area, and positioned such that the light path between them traverses the expected position of the leads as the part moves along the track. By appropriate placement of the source-detector pairs, various different parameters may be measured by examining the output waveforms produced by the detectors as the part goes by. The embodiments described in the Langley and Brekka patent include source-detector pairs adapted to measure parallel and camber bending of the leads. The patent mentions that the sources may be essentially infrared point sources and the detectors may be phototransistors. Alternatively, point sources could be replaced by a light source (such as a laser) whose light is controlled to intersect an entire lead, and detectors replaced by a linear photodiode array. As another alternative a light source may be used which illuminates an entire side of leads while the part under test remains stationary. A sensor may be a TV camera that electronically scans the resulting picture of the leads.
This technique is highly versatile and is capable of high accuracy at high throughputs. However, after much time and effort was spent in attempts to increase the accuracy and/or throughput to still higher levels, it was finally determined that such increases are limited by the need for transport means which are steady enough to prevent unwanted motion when the part passes through the test area. Alternatively, it was found very difficult to hold the part steady and move the light sources and detectors without introducing unwanted motion of the part.